Science

Here’s everything you need to know about compost

Compost can be fun, it can be smelly, it can be frustrating. Most of all, it can be a really good way to feed your food scraps and other organic waste back into growing plants to produce more food from your garden, while reducing landfill and our greenhouse gas emissions.

But compost can appear mysterious, and requires some trial and error. In celebration of National Compost Week – a jubilee of all things soil, scraps, smell and salvaging leftovers we didn’t know we needed – Cosmos consulted the experts to give us the rundown on what compost is and to give us some composting dos and don’ts.

What is compost?

Compost is part of “the lifecycle of soil”, explains Emily Bryson, a PhD student in environmental science at Central Queensland University.

“Plants take up nutrients from the soil to grow,” Bryson said. “If we keep growing things, we keep taking stuff out of the soil. To keep growing things sustainably, we have to put those nutrients back into the soil. Compost is how we do that.”


More on compost: Odd Jobs: Dr Dog Poo


It’s a science, explains Matthias Salomon, a PhD student at the University of Adelaide. “It’s a natural process. You don’t need to spend a lot of time or money to compost. Theoretically you just need any sort of organic waste products, and you can make perfect compost.”

The composting process is mainly produced through natural microbial biodegradation of organic matter. While this does happen in nature, it happens slowly, says Bryson. Composting techniques can speed this process up in a controlled way by “combining a bunch of organic materials to break them down. The nutrients are locked up in the food waste, but first you need to break it all down so that plants can use it.”

That waste is broken down by all sorts of critters, including bacteria and fungi. “Microbes like to eat, and they start with something that’s easily consumable like sugars,” explains Salomon. The activity of the feasting microbes uses water and generates heat. That heat, in turn, helps to break down starches like cellulose.

Now the fun part. I hope you’re not eating.

“The microbes poop and then other microbes say, ‘Oh, I love your poop, so I’m going to eat that waste and then make my own waste’,” says Bryson. When the mixture starts to cool, microbes break down the hardier materials like the lignin from wood and paper. “In the end, what you’re left with is this dark material that’s kind of sticky and a bit crumbly. It’s the building block for plant and soil nutrients.”

Composting tips

  • Get the ingredients right

Paper, cardboard, sawdust and shredded paper are high in carbon, says Emily Bryson. Plus they can absorb moisture. “When you compost your food scraps, it’s a slimy disgusting mess. We need to balance that with something and that’s the carbon. It will help aerate things as well so you don’t get that stinky anaerobic ammonia smell.”

  • You can compost many things, it just depends on the system you’re using

Bryson reassures me that you can compost onions, citrus and dairy. “It is very much trial and error. If you go to use that compost later and you still have some citrus peels, for example, pull that stuff out and give it another go.” Spent coffee grounds are a Salomon favourite. He is a “strong advocate” because coffee grounds are high in nitrogen and don’t smell.

  • Create a “closed nutrient circle”

Other great compostables are grass clippings, woodchips, and other organic scraps produced in your garden. “This way, you can make sure that you have a closed nutrient circle, where you use your local waste products, compost it, and bring it back into your garden,” says Salomon.

  • Avoid composting meat, fish and bones

These attract vermin, warns Salomon, and they can harbour potentially deadly pathogens like salmonella or listeria. However, some composting methods can deactivate the pathogens making the compost safe for human use.

  • Avoid heavy metal contaminants

This seems obvious. “But this is especially an issue with composite wood chips that have been treated with any sort of antibacterial products treated with antimicrobial agents,” Saloman cautions.

  • If you live in an urban area and are lucky enough to have an organics bin in your local council, use it!

Check your council website to see what they do and don’t accept before you put stuff in it, says Bryson.

  • Be careful with compostable plastic

“Read the labels if you’re going to put any sort of compostable plastic in there,” warns Bryson. “Make sure it says the word ‘compostable’ and then check to see if it’s industrial compostable. Make sure that it doesn’t just say ‘biodegradable’, because that won’t work.”

Composting mistakes

  • Overfeeding your compost bin

“At some point, pick a day to stop feeding your compost bin or pile,” Bryson says. “It’s really tempting to just keep putting stuff in, because when you start feeding it, you can see it breaking down and the volume gets lower. But at some point, stop feeding it new material and let it finish.”

  • Your compost pile or bin is too wet or too dry

Overly wet compost will lose nutrients, especially those that are water soluble like ammonium and nitrates, warns Salomon. Saturated compost can also convert nitrogen sources to ammonia making it stinky. “If it’s too wet, it’s also not ideal for the microbial activity,” he adds.

Too dry and you just don’t have enough moisture content to keep the microbes happy. The Goldilocks zone? “That’s when you squeeze the compost in your hand and a few drops of water slip through your fingers.”

  • Burying organic waste in the ground and waiting

“Just burying organic stuff isn’t composting,” laughs Bryson. “You need to be prepared to give it some attention by turning it to aerate it. Some people think that composting means you can set it and forget it, but you need to invest a bit of time.”

“Some people see steam coming out of compost after a few weeks and think it’s ready to use, but this is the maturation phase,” explains Salomon. “In that case it would be best to compost another couple of weeks to allow the microorganisms in the compost to reach some equilibrium.”

At the end of the day, composting is an important part of life in the modern world and is a pretty forgiving process, says Bryson. “If things start to go awry, you can always add more stuff and try and correct those things. It’s very much trial and error.”

Composting myths

  • Compost is the same as fertiliser

No! Bryson says compost should be thought of as a “soil conditioner” first. “It builds soil structure – it creates the substrate, the canvas for all the fun things to happen, and helps hold moisture.

“Fertiliser is any sort of nutrient that you add to a plant that isn’t there in nature already. There are nutrients in compost, depending on what you put in it, and how mature it is. But it’s not the same as fertiliser. Compost provides an ecosystem that will hold fertiliser and water.”

  • You can’t compost dog poo

This one hit close to home for Emily Bryson, as it’s the topic of her PhD research. People think dog poo is a risky addition to compost because dogs eat meat, and it’s not recommended to compost meat scraps. “But there’s no evidence that omnivorous animals have less bacteria in their gut. In fact, we know that cows, pigs, horses, and chickens can have bacteria in their gut that can be harmful to humans. All animal manure needs to be processed before it can be used.”



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